The art or formulating lubricating oil compositions has become more complex as a result of increased government and user environmental standards and increased user performance requirements. Hydraulic fluids are typically marketed based upon features such as fluid durability, deposit control, antiwear protection, filterability, water tolerance, rust/corrosion protection, and viscosity.
Viscosity Modifiers (“VM”) are polymers for example, polymethacrylate (“PMA”) which are typically used in lubricants including hydraulic fluids to alter the viscometric properties of the lubricant. This usually takes the form of increasing the viscosity index (“VI”) of the fluid, resulting in improved low temperature properties and increased viscosity at higher temperatures.
The performance of the PMA viscosity modifier is dependant upon its molecular weight (“MW”). The high MW (“HMW”) polymers provide better VI improvements and conversely, lower temperature properties & higher viscosities at elevated temperatures than the low MW (“LMW”) materials at equivalent treat rates.
These HMW polymers however, have one disadvantage, poor shear stability. The HMW lose their ability to provide the increased viscosity at higher temperatures in a high shear environment for example a vane pump.
This poor shear stability or loss of viscosity at operating temperature can lead to reduced equipment life due to increased wear. It can also lead to a reduction in overall pump efficiency, leading to increased operating costs.
In summary, VM performance is related to its molecular weight. The higher the MW, the better the VI lift & thickening efficiency of the polymer, the lower the molecular weight the better the shear stability.
Accordingly, there is a need for improved shear stability in industrial oils without comprising the thickening efficiency of the polymer. This invention satisfies that need.